1. Preheat the oven to 350°.
2. Coat the bottom of a 9″ baking dish with the olive oil and arrange the mushroom caps stem side up.
3. In a large skillet over medium heat, cook the Italian sausage, onion, bell pepper, and garlic until the sausage is browned and the veggies are beginning to soften. Season with the salt. Toss in the spinach until it begins to wilt.
4. Remove the sausage mixture from the heat and stir in the diced cheese.
5. Mound the sausage mixture into the mushroom caps, pressing to firm.
6. Bake the stuffed mushrooms in the pre-heated 350° oven for 25 minutes, or until the mushrooms are tender and the filling is browned.